Low solar absorptance, high emissivity, inorganic electrostatic dissipative thermal control coating

ABSTRACT

An electrostatic dissipative paint including a plurality of particles having a composition selected from the group consisting of gallium magnesium oxide, gallium aluminum magnesium oxide and combinations thereof. The paint further includes an inorganic binder mixed with the particles to form a mixture. A method of making a thermal paint, a method of applying thermal paint and painted components are also disclosed.

FIELD OF THE DISCLOSURE

The present disclosure is directed to thermal control coatings andcoating composition, and, more particularly, to a white paint useful forcoating spacecraft and components of spacecraft.

BACKGROUND

Spacecraft are subjected to a wide range of thermal environments duringservice. For example, one side of the spacecraft may face in a directionaway from the sun, while another side faces toward the sun. Thermalcontrol is desirable because heat is radiated into space, which coolsthe spacecraft, but the spacecraft can simultaneously be heatedintensively in direct sunlight.

Active and passive temperature control techniques are used to maintainthe interior temperature of the spacecraft, which contains persons orsensitive instruments, within acceptable operating limits. Activetemperature control may involve machinery or electrical devices, such aselectrical heaters, electrical coolers, and heat pipes. In contrast,passive temperature controls are techniques that do not involvemachinery or electrical devices, but may include thermal coatings orstructural designs.

Specifically, one known approach to passive temperature control includesuse of surface coatings, typically termed “paints”, on the externalsurface of the spacecraft. A white paint, for example, has a low solarabsorptance, while a black paint has a high solar absorptance. Selectiveapplication of such paints to various elements of the spacecraftexterior greatly aids in controlling its temperature.

In addition to passive temperature control, it is desirable for paintapplied to the surface of spacecraft to dissipate electrostatic charges(i.e., provide electrostatic dissipation (ESD)) that may develop alongthe external surface of the spacecraft. The electrostatic charges mayaccumulate and cause arcing and possible damage to, or interferencewith, sensitive electronic equipment on or in the spacecraft. In orderto dissipate electrostatic charge, the paint must have at least someelectrical conductivity. Specifically, it is desirable that coatingscapable of electrostatic dissipation (ESD) have a surface resistivity ofless than about 10⁹ ohms per square.

In addition to thermal control and ESD, paint for use on spacecraft andspacecraft components should exhibit additional characteristics forspacecraft applications. For example, the paint should be stable duringlong-term service in a space environment. The paint is desirablymoderately tough and flexible so that it does not crack and flake awayas it is flexed due to mechanical or thermal strains.

A number of white, electrostatic-dissipative paints are known forspacecraft use. One of the known paints includes an inorganic potassiumsilicate binder. The paint having the potassium silicate bindertypically has a solar absorptance of from about 0.13 to about 0.15, seeU.S. Pat. No. 5,094,693, whose disclosure is incorporated by referencein its entirety. However, it is desirable to have paints with lowersolar absorptance. The lower the value of the solar absorptance, thelower the heating of the paint and thence the underlying substrate, inthe intense heating of direct sunlight.

Known white thermal paints have high production costs. For example,doping of materials involves labor-intensive and expensive processes andexpensive materials.

What is needed is an improved white thermal-control paint that isoperable and stable in a space environment, which has a lower solarabsorptance than available in existing paints, which has a loweroperating temperature-limit than existing paints, which can manageelectrostatic discharge (ESD) and can be manufactured inexpensively. Inaddition, it is desired to have an inorganic white thermal controlcoating which; a) minimizes space radiation degradation over time, b)minimizes the beginning of life (BOL) and the end of life (EOL) solarabsorptance (α), c) maximizes the infrared emissivity (e), while at thesame time d) maximizing the electrostatic dissipative (ESD) propertiesat low temperatures (below −65° C.). The present disclosure fulfillsthis need, and further provides related advantages.

SUMMARY

One aspect of the present disclosure is a thermal paint including aplurality of particles having a composition selected from the groupconsisting of gallium magnesium oxide, gallium aluminum magnesium oxideand combinations thereof. The paint further includes an inorganic bindermixed with the particles to form a mixture.

Another aspect of the present disclosure is an electrostatic dissipativecoating having a plurality of particles having a composition selectedfrom the group consisting of gallium magnesium oxide, gallium aluminummagnesium oxide and combinations thereof and a cured inorganic binderhaving the particles incorporated therein.

Still another aspect of the present disclosure is a method for making athermal paint. The method includes providing gallium oxide powder and amagnesium oxide powder. The gallium oxide powder and a magnesium oxidepowder are mixed and calcined at 1100° C. in an atmosphere of 10 vol %hydrogen with the balance argon for a sufficient time to produce apigment having a spinel structure. The method optionally includesannealing the calcined powder at 800° C. in an atmosphere of 10 vol %hydrogen with balance argon. The method further includes incorporatingthe pigment into a binder to form a paint.

Still another aspect of the present disclosure is a method forprotecting a spacecraft. The method includes providing a spacecraftcomponent. A coating is applied to a surface of the component. Thecoating contains a pigment selected from the group consisting of galliummagnesium oxide, gallium aluminum magnesium oxide and combinationsthereof.

One advantage includes reduced production costs in comparison to knownwhite thermal paints utilizing doped materials.

The paint will help provide longer spacecraft lifetimes and increasedpower usage for enhanced communications capabilities. In addition, theimproved physical properties of this thermal control paint should leadto improved designs, increased lifetime, and increased power usageabilities of spacecraft, including, but not limited to satellites.

Another advantage is that the pigment is self-doping and electricallyconductive, wherein time-consuming and labor-intensive doping of thematerial followed by expensive precision elemental analyses is notrequired.

Still another advantage is that the exclusion of zinc from the pigmentmaterial may provide increased resistance to exposure to radiation incomparison to zinc containing materials. In addition, the specificweight of this coating is lower than known coatings, leading to lighterspacecraft components.

The pigment has been shown to provide electrostatic dissipativedischarge (ESD) properties to −170° C. while in the form of the desiredcoating, or perhaps the coating has been shown to provide ESD protection(or properties) to −170° C.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a paint layer according to anembodiment of the disclosure.

FIG. 2 is a side elevational view of a paint layer applied to asubstrate according to an embodiment of the present disclosure.

FIG. 3 is a perspective view of a spacecraft having a paint layeraccording to the disclosure.

FIG. 4 is a block diagram of a method for the preparation of a whitepaint according to the disclosure and the painting of a substrate.

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

DETAILED DESCRIPTION

The present disclosure includes paints and coating systems incorporatingan electrically conductive pigment. The pigment is made up of particleshaving spinel structure of gallium magnesium oxide or magnesium gallatehaving the following structure:

Ga_((2+x))MgO_(4-δ)

wherein x is a value from about 0 to 0.1 and δ is in the range of about0.01 to about 0.0001.

Another embodiment of the present disclosure includes gallium aluminummagnesium oxide or aluminum magnesium gallate:

Ga_((2+x−y) Al) _(y)MgO₄₋₆

wherein x is a value from about 0 to 0.1 and δ is in the range of about0.01 to about 0.0001 and y is from 0 to about 0.10 and δ is from 0.01 toabout 0.0001.

The term “spinel” as used herein includes spinel structures arranged ina face center cubic lattice, wherein the spinel generally follows thefollowing formula AB₂ O₄. In the “normal” spinel arrangement of atoms,the A includes one or more divalent cations that occupy tetrahedralsites and the B includes one or more trivalent cations that occupy theoctahedral sites. In the “inverse” spinel arrangement of atoms, the Aincludes one or more atoms that occupy the octahedral sites whileone-half of the B atoms occupy the tetrahedral sites. While not wishingto be bound by theory, it is believed that the formulas Ga_((2+x))MgO_(4-δ), and Ga_((2+x−y))Al_(y)MgO_(4-δ) include both the normal andinverse structures. This is known as a “mixed” spinel arrangement ofcations.

The pigment according to certain embodiments of the disclosure includenon-stoichiometric solid solutions of gallium magnesium oxide(Ga_((2+x)) MgO_(4-δ)) having slight stoichiometric excess of galliumoxide. While not wishing to be bound by theory, the electricalconductivity of the pigment material is believed to be a result ofexcess stoichiometric gallium or stoichiometric deficiency of magnesiumpresent in the spinel structure. It is further believed that at leastsome portion of a rare valence state of Ga2+ cations occupy both theoctahedral sites and the tetrahedral sites permitting the exchange ofelectrons from the Ga²⁺ to the Ga³⁺ cations on both the tetrahedral andoctahedral sublattices. The excess gallium (i.e., gallium oxide) in thespinel structure affects an essentially self-doping material, whichresults in electrical conductivity. The electrical conductivity makesthe pigment suitable for use in applications, such as electrostaticdissipation (ESD), which require electrical conductivity.

FIG. 1 illustrates a paint layer 100 prepared according to thedisclosure. The paint layer 100 comprises particles 103 mixed with abinder 105. The particles include pigment, and, optionally, activesecondary particles and/or inert secondary particles. Prior to dryingand/or curing, a paint vehicle is also present, but the paint vehicle isvolatilized during the drying/curing operation.

FIG. 2 illustrates the paint layer 100 applied to a substrate 201. Thepaint of the present disclosure may be utilized on a variety ofsubstrates. Suitable substrates include metallic substrates comprisingaluminum, aluminum alloys, titanium, titanium alloys, nonmetallicmaterials and composites such as graphite/epoxy and graphite/polycyanateester composites. In particular, the paint of the present disclosure issuitable for use upon spacecraft, such as satellite radiator coatings,microwave antenna coatings, and sun-shields.

The paint of the disclosure may be used in any thermal controlapplication. In one embodiment, the paint is used as a coating on aspacecraft 300, such as a satellite illustrated in FIG. 3. Thespacecraft 300, here depicted as a communications satellite that ispositioned in geosynchronous orbit when in service, has a body 303 withsolar panels 305 mounted either on the body 303 or on wings 307 thatextend outwardly from the body 303, or both. The body 303 and wings 307have a skin 309 which may be made of a metal, a nonmetal, or a compositematerial, and which may be supported by an underlying skeletalstructure. At least some of those outwardly facing portions of the skin309 of the body 303 and/or the wings 307, which may or may not comprisesolar panels are covered with the layer 100 of the paint of thedisclosure, as described above. The skin of the spacecraft therebyserves as the substrate 201 to which the paint layer 100 is applied. Thepaint layer 100 provides the covered portions with passive thermalcontrol and electrostatic charge dissipation capabilities. The paint issufficiently durable and stable in its properties for use on extendedmissions of 15 years or more.

To form the paint according to embodiments of the disclosure, thepigment is mixed with a suitable binder. Binders useful for formingpaint according to an embodiment of the disclosure include inorganicbinders. Examples of suitable binders include, but are not limited topotassium silicate solution (e.g., KASIL® 2135). KASIL® is a federallyregistered trademark of PQ Corporation, Valley Forge, Pa. Other suitableinorganic binders may also include sodium-potassium silicates. Thepigment to binder ratio (PBR) may include ratios of about 3.5 to 1 orfrom about 2.5 to 1 to about 4.0 to 1.

The pigment is formed from calcining gallium oxide and magnesium oxidepowder with an excess of gallium oxide at 1100° C. in an atmosphere of10 vol % hydrogen with balance argon to form a spinel structure andsubsequently annealing the material at 800° C. in an atmosphere of 10vol % hydrogen with balance argon. “Calcining”, and grammaticalvariations thereof, as utilized herein, are high temperature processesapplied to a solid material in order to bring about a thermaldecomposition, and/or phase transition, wherein the conditions are suchthat a solid solution spinel structure is formed.

FIG. 4 depicts one embodiment of a method for preparing particles 103wherein the pigment particle composition is Ga_((2+x)) MgO_(4-δ), wherex is a value from about 0 to 0.1 and δ is in the range of about 0.01 toabout 0.0001. The pigment is utilized in the paint material used in thepaint layer 100, and for painting the substrate. As shown in FIG. 4, themethod includes the steps of making the pigment, making the paint andapplying the paint. In making the pigment, the raw materials (e.g.,gallium oxide and magnesium oxide) are provided, step 400, 401 andweighed, step 403. As discussed above, the ratios of the components areweighed to provide the desired ratios with respect to stoichiometry.Specifically, certain embodiments include a stoichiometric excess ofgallium oxide. The raw materials are milled together to form a mixture,step 405. The milling is accomplished in a milling medium. The millingmedium may be a suitable organic carrier, such as hexane. After millingis complete, the mixing medium is removed by drying or otherwisevolatilizing the mixing medium, step 407.

The dried mixture is fired (i.e., calcined) to chemically react thecomponents together, step 409, at a temperature that in one embodimentis in the range of from about 1000° C. to about 1300° C. Certainembodiments include a firing treatment from about 2 hours at highertemperature to about 24 hours at lower temperature. In one embodiment,the firing treatment takes place at a temperature of 1100° C. for sixhours, in an atmosphere of 10 vol % hydrogen with the balance argon. Themixture is thereafter optionally annealed, step 411, after the firing ata lower temperature, for example 1000° C. for several hours. Forexample, the annealing may take place for from about 1 to 10 hours. Inan alternate embodiment the method may include a slow cooling ramp-down,rather than annealing at a constant temperature. The optional annealingaffects the degree of spinel disordering. The resultant mass is bothaggregated and agglomerated.

The pigment formed is prepared for formulation of a paint. In order tomake the paint the pigment from step 411 is provided, step 413. Water orother suitable vehicle is also provided, step 417. The pigment from step413 and the vehicle from step 417 are weighed and batched, step 419.Thereafter the pigment and vehicle are milled together, step 421. Theresultant particulates have a size range from about 5 micrometers toabout 25 micrometers.

Subsequent to milling, binder material is dispersed into the milled,particulate solution, step 423. There may be some separation overextended periods of time, but the paint is normally stirred or agitatedjust before or at the time of application. A binder is provided, step415, to adhere the particles together in the final product. The binderis selected to provide good adherence of the particles to each other andof the particles to the underlying substrate, with acceptable physicalproperties. The binder must withstand the environment to which the paintis exposed, such as a space environment. The binder is present in anoperable amount. In a typical case, the binder is present in an amountsuch that the ratio, by weight, of the total of all of the particulatesto the binder is about 4:1 or less. If the ratio is more than about 4:1,the critical pigment volume concentration (CPVC) may be exceeded, thepaint has insufficient mechanical strength, and the paint falls apartwhen dried. In one embodiment, the ratio by weight of particles tobinder is from about 3:1 to about 4:1 or from about 2.5:1 to about4.0:1. The preparation of the paint is complete, step 425.

Returning to FIG. 4, the paint is used by providing the substrate 201 tobe coated (see FIG. 2), step 427, and cleaning the substrate, numeral429. The substrate 201 may be any suitable substrate capable ofreceiving the coating. There is no known limitation on the type ofsubstrate. The surface of the substrate is cleaned by any operabletechnique, such as washing and scouring in a detergent solution, rinsingin tap water, rinsing in de-ionized water, and drying in air. The paintis applied to the surface of the substrate, step 431. At the outset ofthe application, the surface of the substrate may be primed to improvethe adhesion of the paint. Priming is preferred for application of thepaint containing an inorganic binder to metallic surfaces, such asaluminum. Preferably, the priming, if used, is accomplished by rubbing asmall amount of the paint into the surface using a clean cloth, toachieve good contact to the surface.

The paint layer is thereafter applied by any operable technique,including, but not limited to spraying, rolling, or brushing.

As the liquid paint dries, curing of the binder occurs. The paint iscured, as necessary, to leave a thin film of a solid material, step 433.Curing may be accomplished at ambient temperature and ambient humiditywithin approximately 5 days. A 50 percent or greater humidity and for atime of about 14 days is one preferred schedule. Drying removes thepaint vehicle by evaporation. Additionally, the drying step mayaccomplish a degree of curing of any curable components, as where acurable inorganic binder is used. The paint layer is preferably fromabout 0.003 to about 0.006 inch in thickness or from about 0.003 toabout 0.006 inch in thickness.

In addition to the described Ga_((2+x))MgO_(4-δ) pigment particles,wherein x is from about 0 to about 0.1, and 6 is from about 0.01 to0.0001, the material may contain active or inert components to modifythe properties of the paint. For example, properties such as opticalproperties and/or the mechanical properties of the final material may bealtered by adding additional active or inert components. In oneembodiment, active secondary particles interact optically with incidentenergy, and may include, for example, aluminum-doped zinc oxideparticles such as described in U.S. Pat. No. 5,094,693, which isincorporated herein by reference in its entirety. Such active secondaryparticles may be utilized to improve the low temperature electricalconductivity at the expense of optical properties, for particularapplications. Inert secondary particles are those which serve primarilyas filler to increase the volume fraction of particulate materialpresent without greatly modifying the optical properties. The inertsecondary particles may be added for economic reasons, as they are oflower cost than the pigment particles, and active secondary particles.Inert secondary particles can include, for example, barium sulfate,clay, or talc.

The coatings according to embodiments of the present disclosure do notcontain the elements zinc or indium. Zinc and indium are believed to bedetrimental to space radiation hardness over time, particularly whenutilized in inorganic ESD coatings. The infrared emissivity of thiswhite coating is very high and just slightly lower than in blackcoatings. This leads to more efficient heat dissipation for satelliteradiators. The beginning of life (BOL) solar absorptance (α) is lowerthan known coating systems, including doped material systems, per unitcoating thickness. In addition, the infrared emissivity (e) of the newcoating is greater than known inorganic white-coating systems. Forexample, in one known system includes a value for e of 0.89 wherein thecoating system according to one embodiment includes a value for e of0.95. As such, for example, the coating system according to embodimentof the present disclosure is more efficient at dissipating heat forsatellite radiators.

EXAMPLES

TABLE 1 Coating BOL Solar BOL Infrared EXAMPLE # Thickness (mils)Absorptance (α) Emissivity (e) Example 1 4.4 0.0689 Example 2 4.7 0.0678Example 3 2.4 — 0.949 Example 4 2.7 — 0.944

Example 1 a paint applied to a thickness of 4.4 mils on an aluminumalloy substrate. The BOL solar absorptance of Example 1 is α=0.0689.Example 2 includes a paint thickness of 5.7 mils as applied to analuminum alloy substrate, BOL α=0.0678. Example 3 includes a paintthickness of 2.4 mils applies to an aluminum alloy substrate, BOLe=0.944. Example 4 includes a paint thickness of 2.7 mils as applied toan aluminum alloy substrate, BOL e=0.955.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. An electrostatic dissipative paint comprising: a plurality ofparticles having a composition selected from the group consisting ofgallium magnesium oxide, gallium aluminum magnesium oxide andcombinations thereof; and an inorganic binder mixed with the particlesto form a mixture.
 2. The paint of claim 1, wherein the pigmentcomprises a composition having the following structure:(Ga_((2+x))MgO_(4-δ)); wherein the value of x is a value from about 0 toabout 0.1 and the value of δ is from about 0.01 to 0.0001.
 3. The paintof claim 1, wherein the binder is potassium silicate.
 4. The paint ofclaim 1, wherein the paint is electrically conductive.
 5. Anelectrostatic dissipative coating system comprising: a plurality ofparticles having a composition selected from the group consisting ofgallium magnesium oxide, gallium aluminum magnesium oxide andcombinations thereof; and a cured inorganic binder having the particlesincorporated therein.
 6. The coating system of claim 5, wherein thepigment comprises a composition having the following structure:(Ga_((2+x))MgO_(4-δ)); wherein the value of x is a value from about 0 toabout 0.1 and the value of δ is from about 0.01 to 0.0001.
 7. Thecoating system of claim 5, wherein the binder is potassium silicate. 8.The coating system of claim 5, wherein the cured inorganic binder havingthe particles incorporated therein is electrically conductive.
 9. Thecoating system of claim 5, wherein the coating system further includesthe cured inorganic binder disposed on a substrate.
 10. The coatingsystem of claim 9, wherein the substrate includes a spacecraftcomponent.
 11. The coating system of claim 10, wherein the spacecraftcomponent includes a satellite.
 12. A method for making a thermal paintcomprising: providing gallium oxide powder; providing a magnesium oxidepowder; mixing the gallium oxide powder and the magnesium oxide powder;calcining the mixture at 1100° C. for a sufficient time to produce apigment having a spinel structure; optionally annealing the calcinedpowder at 800° C. in an atmosphere of 10 vol % hydrogen with balanceargon; and incorporating the pigment into a binder to form a paint. 13.The method of claim 12, wherein the pigment comprises a compositionhaving the following structure:(Ga_((2+x))MgO_(4-δ)); wherein the value of x is a value from about 0 toabout 0.1 and the value of δ is from about 0.01 to 0.0001.
 14. Themethod of claim 12, wherein the binder is potassium silicate.
 15. Themethod of claim 12, wherein the paint is electrically conductive.
 16. Amethod for protecting a spacecraft: comprising providing a spacecraftcomponent; and applying a coating to a surface of the component, thecoating containing a pigment selected from the group consisting ofgallium magnesium oxide, gallium aluminum magnesium oxide andcombinations thereof.
 17. The method of claim 16, wherein the pigmentcomprises a composition having the following structure:(Ga_((2+x))MgO_(4-δ)); wherein the value of x is a value from about 0 toabout 0.1 and the value of δ is from about 0.01 to 0.0001.
 18. Themethod of claim 16, wherein the binder is potassium silicate.
 19. Themethod of claim 16, wherein the applying comprises spraying, rolling, orbrushing.
 20. The method of claim 16, wherein the spacecraft componentis a satellite.
 21. The method of claim 16, wherein the coating iselectrically conductive.